Unidirectional
Alternatively referred to as Standard Parallel Port (SPP), unidirectional refers to one direction where data is either sent or received. For example, the parallel port on a computer can be set to unidirectional allowing the computer to send data to a device such as a parallel port printer but not allow that printer to communicate back to the printer.
Unidirectional ports were found with early computers manufactured before 1995, capable of transmitting data 4-bits at a time, transmitted data at 40 to 60 KBps, and capable of using a 6 foot cable. This standard was latter moved into the IEEE-1284, updated, and now known as Centronics mode.
Also see: Bidirectional, Centronics mode, Parallel port, Printer definitions